1. Field of the Invention
The invention relates to a method for determining and displaying spectra for vibration signals. In particular, the invention relates to a method for determining and displaying spectra for structure-borne sound signals which are emitted by machines with rotating or oscillating components, in which a plurality of measurement signals are detected successively in time, individually or as a data set, by means of data acquisition means, in particular in the form of portable data gatherers, signal analyzers or permanently installed continuous monitoring devices, at at least one measurement point by means of a vibration sensor on a machine, and are fed to an evaluation unit in a vibration measurement device or in an evaluation station, and are subjected by means of the evaluation unit to frequency transformation, specifically to a discrete Fourier transformation with a constant absolute narrowband bandwidth, and in which case the results of such a transformation are ordered on the basis of individual frequencies and are reproduced on the basis of magnitude, phase or complex-value results.
2. Description of Related Art
A method of the type to which the present invention is directed is known from the book Machinery Analysis and Monitoring by John S. Mitchell (second edition, Pennwell Publishing Company, Tulsa, Okla., ISBN 0-87814-401-3, 1993). Pages 107 to 110 in this book describe the advantages resulting from the use of data gathering devices (so-called data gatherers) when they are used to detect structure-borne sound measurement variables. Such structure-borne sound emissions are produced, in particular, by rotation, and thus, vibrating machines or machine parts. The monitoring of vibration states of such machines is made considerably more economic and more efficient overall by the use of suitable signal diagnostic devices. This is particularly true if the data gatherers can be combined with a superordinate data evaluation station (for example, in the form of a personal computer). This provides good capabilities for also measuring a relatively large number of machines and their machine parts periodically in one cycle of a measurement round (possibly lasting several hours), and thus, allows their status to be monitored continuously, and if necessary, to be analyzed in more detail when significant changes occur. In this process, detected measurement variables and signals are, first of all, buffer-stored in a data gatherer. The multiplicity of detected measurement variables are then transmitted to a memory in the superordinate data evaluation station, and can then be combined, displayed and evaluated in a relatively convenient manner by means of associated software.
Overall, such a procedure has the object of preventing damage to machines, or at least of identifying such damage at an early stage. In this context, it may be sufficient to carry out a check of machine vibration, for example, with respect to level magnitude and characteristics of the bearing noise or rotor vibration, in cycles of only once a week or once a month.
In the meantime, portable data acquisition devices (so-called FFT data gatherers) are able to buffer-store a considerable number of individual readings and measurement signals. This has thus made it possible to carry out the checking of machine vibration by means of high-resolution time signal investigation and spectral analyses. For example, to this end, a data set of about 10,000 to 20,000 individual samples of the vibration signal is, first of all, buffer-stored for an individual machine check and, in a second step, is subjected to so-called Fourier transformation. The result of the Fourier transformation can then be displayed on a screen or a visual display unit, in a manner known per se.
Currently, a plurality of signals, each with different measurement settings, are measured successively at one measurement point corresponding to a component to be diagnosed, in order to produce the overall state investigation. To do this, the vibration signal for a component to be investigated individually must be analyzed with the correspondingly required frequency resolution in the frequency band in which the component vibration occurs. It is well known that lower-frequency vibration occurs as the size of the various components and assemblies increases, which vibration is measured, at relatively low frequencies, as an oscillation rate or as an oscillation displacement, in order to achieve a good signal dynamic response and a good signal-to-noise ratio. The frequency resolution required for the individual narrowband lines becomes finer for relatively low frequencies, in order to make it possible to distinguish between the individual vibration components. For example, the measurement settings for vibration signals from large, slow rotors must be designed such that the oscillation displacement in a frequency band from 2 to 400 Hz is analyzed with a resolution of about 0.1 Hz, that is to say 3200 lines. The vibration signals from small components, such as roller bearings, in contrast, are analyzed with sufficient accuracy using an oscillation acceleration in a frequency band of from 1 kHz to 40 kHz with 3200 lines, each with a resolution of 12.5 Hz. If attempts are now made, for simplicity, to analyze only one vibration signal for all components, up to a high frequency, then the resolution is insufficient for the rotors, even with far greater numbers of lines.
In order to detect the frequency spectra of vibration signals, oscillation acceleration sensors are fitted, in a known manner, at a chosen measurement position, and these oscillation acceleration sensors supply the vibration signal, which is amplified in the measurement device and is filtered by hardware, in the frequency domain. This is followed by A/D conversion of the signal with sampling at a sampling frequency which, as is known, must be about twice the highest cut-off frequency to be investigated in the frequency spectra in order to allow the digital time signal to be used to calculate a frequency spectrum correctly, by means of an FFT or other transformation. A correspondingly large number of samples must be provided in order to obtain the required frequency resolution of the individual narrowband lines in the frequency band. Frequency bands with a constant absolute bandwidth are used for machine vibration signals.
A portion of an illustrated frequency band can be formed, for an advantageous display, from the measured frequency band now obtained. If it is intended to investigate a large number of machines at a plurality of measurement points in one measurement cycle or over a long period of time more than once, then an appropriately large amount of memory must be provided. It is self-evident that a larger memory results, in a disadvantageous manner, in a larger physical volume and increases the cost of a signal diagnostic device. So-called analyzers are used in a similar manner to data gatherers, no periodic monitoring being carried out in this case, but a more far-reaching cause analysis being carried out, only if required, if the vibration states change. On machines which are, for example, very important or to which access is difficult, the task of the data gatherer can also be carried out in the same manner by a permanently installed monitoring device, with continuous monitoring. However, measurement variables or signals are then detected continuously and, if required, are processed further, finally stored, evaluated and visualized, as required, via a relatively long data line, which is likewise permanently installed, at the evaluation station. The FFT data gatherers, analyzers and permanently installed monitoring devices are also referred to as signal diagnostic devices in the following text.